Lithium ion secondary batteries are second batteries having high charge and discharge capacity and capable of outputting high power. The lithium ion second batteries are now the dominant power source for portable electronic devices and considered to be a promising power source for electric vehicles to be widely used in future. A lithium ion secondary battery has an active material capable of absorbing and releasing lithium (Li) at each of a positive electrode and a negative electrode. The lithium ion secondary battery works by moving lithium ions in an electrolytic solution provided between the electrodes.
In the lithium ion secondary battery, lithium-containing metal composite oxide such as lithium-cobalt composite oxide is mainly used as an active material for a positive electrode, and a carbon material having a multilayer structure is mainly used as an active material for a negative electrode. Performance of the lithium ion secondary battery depends on raw materials of the positive electrode, the negative electrode, and an electrolyte constituting the secondary battery. Research and development are now actively carried out particularly on raw materials of the active materials. For example, silicon or silicon oxide having a higher capacity than carbon is being studied as a raw material for the negative electrode active material.
A battery having a higher capacity can be obtained by using silicon as the negative electrode active material than by using carbon materials. Silicon, however, has a large volume change associated with Li absorption and release in electric charge and discharge. Therefore, silicon readily changes into fine powder and drops or peels off from a current collector, so there arises a problem that such a battery has a short charge and discharge cycle life. Upon using silicon oxide instead of silicon as the negative electrode active material, volume change associated with Li absorption and release in electric charge and discharge can be suppressed.
For example, use of silicon oxide (SiOx; about 0.5≦x≦1.6) as the negative electrode active material is being studied. It is known that when subjected to heat treatment, SiOx decomposes into Si and SiO2. This is called a disproportionation reaction and SiOx is separated into two phases of Si phase and SiO2 phase owing to an internal reaction in the solid. The SiO2 obtained by the separation is very fine. In addition, the SiO2 phase covering the Si phase serves to suppress decomposition of an electrolytic solution. Therefore, a secondary battery using SiOx which has been decomposed into Si and SiO2 as the negative electrode active material has good cycle characteristics.
However, even in the abovementioned lithium ion secondary battery using silicon oxide as the negative electrode active material, expansion and shrinkage of the negative electrode in charge and discharge cannot be avoided, so there is a problem that fatigue breakdown of the negative electrode occurs. Furthermore, in the negative electrode of the lithium ion secondary battery, an electrolyte or an electrolytic solution sometimes undergoes reductive decomposition in charge or discharge, and a precipitate produced by the decomposition may deposit to form a coating film. Such a coating film causes problems such as an increase in resistance and a decrease in load characteristics.
Japanese Unexamined Patent Publication No. 2004-185810 mentions that fatigue breakdown can be prevented by forming a negative electrode by using active material particles whose surfaces are coated with a polymer. Moreover, Japanese Unexamined Patent Publication No. 2009-176703 discloses that drop off of active material particles caused by pressing is prevented by coating a surface of a silicon oxide-containing negative electrode active material layer with a polymer. Since formation of such a polymer coating layer prevents direct contact of a negative electrode active material and an electrolytic solution, the formation is also expected to prevent decomposition of an electrolyte or the electrolyte solution.
However, there is a problem that a too small thickness of the polymer coating layer makes it difficult to exhibit the abovementioned effects, while a great thickness of the polymer coating layer increases resistance. The abovementioned patent documents disclose kneading and spin coating as examples of a method for applying a polymer. However, these coating methods are difficult to control coating film thickness, and as a result are difficult to satisfy paradoxical requests of suppression of reductive decomposition of the electrolyte or the electrolytic solution and suppression of an increase in resistance.